ATLANTA, Georgia (CNN) -- CNN was targeted Thursday by attempts to interrupt its news Web site, resulting in countermeasures that caused the service to be slow or unavailable to some users in limited areas of Asia.

CNN took preventative measures to filter traffic in response to attempts to disrupt its Web site.

"CNN took preventative measures to filter traffic in response to attempts to disrupt our Web site. A small percentage of CNN.com users in Asia are impacted," the network said in a statement.

"We do not know who is responsible, nor can we confirm where it came from," the statement continued.

A CNN spokesman said the Web site began to notice problems around midday Thursday and took measures to isolate the trouble by limiting the number of users who could access it from specific geographic areas.

As a result, he said, some users in those areas experienced temporary slowdowns or problems accessing the site.

The spokesman could not offer an estimate of how many users were affected. However, he said that the impact on daily usage was "imperceptible" and that the site "at no time" went down.

Service had returned to normal by mid-morning Friday, he said.

The attempt came as tech-oriented Web sites in Asia were reporting calls from hacker groups in China for denial-of-service attacks to be launched against the CNN Web site on Saturday over the network's coverage of unrest in Tibet.

Angry Chinese bloggers have accused CNN and several other Western news organizations of being unfair in covering recent pro-independence protests in Tibet, which is controlled by China.

In response, the network released a statement noting that "CNN's reputation is based on reporting global news accurately and impartially, while our coverage through the use of words, images or video always reflects a wide range of opinions and points of view on every story."

In a denial-of-service attack, hackers use automated programs to try to jam a site with bogus requests for service to the point that service is slowed or interrupted for legitimate users.E-mail to a friend